In a receiver (RX) lineup having differential circuitry, such as an RFIC for governmental and public safety use, there is often a need for a low power, low noise, local oscillator (LO) feed-through and a high linearity and passive doubly-balanced type mixer. It had been thought that a low noise, high linearity receiver is better designed by using a current-mode topology. In using a current-mode topology, a low-noise amplifier (LNA) has a transconductance amplifier pushing and pulling current through mixer switches into a low input impedance (Zin) of a post-mixer amplifier (PMA). However, since the LNA of the current-mode topology typically has a high output impedance (Zout), such a combination may lead to linearity degradation at high frequencies due to the fact that conductance for the mixer gets smaller (larger resistance) at higher LO frequencies causing the high impedance output stage of the LNA to compress.
One alternative to using the current-mode topology is to use a Voltage-Mode LNA with a low Zout and using a resistance of the mixer as part of a gain-setting resistor pair of the PMA. However, the value of the effective resistance of the mixer is dependent upon the switching frequency, and the higher the value of the effective resistance is, the lower the overall gain of the PMA. In order to overcome this problem, any switching resistance of the mixer could be made insignificant by inserting a large-enough series resistor in between the mixer and the input of the PMA, such that the gain of the PMA no longer is dependent upon the mixer. As a result, if the LO of the mixer needs to vary over a very wide band, it is better to use a ‘voltage-mode’ design with low Zout LNAs driving passive mixers which have near zero current through them into a high Zin PMA.
It is also desirable for the RX lineup to have very low noise. This leads to avoiding the use of an inverting op-amp configuration in the PMA as a very large resistor would be used to realize a large Zin, thereby causing a significant amount of thermal noise. Furthermore, if bipolar transistors are utilized at the PMA inputs to ensure low close-in noise, the transistor base current results in DC current through the resistors connected to the PMA inputs, which results in an additional source of noise (flicker noise). A large resistor also uses an extensive amount of surface area on the semiconductor wafer on which the PMA is fabricated.
One alternative way to implement a fully-differential non-inverting PMA configuration uses an op-amp topology having dual-input differential pairs. However, this op-amp topology has several weaknesses. First, the extra active input devices add uncorrelated noise in the signal path. Second, a Common-Mode Feedback (CMFB) structure is present in the op-amp topology. The CMFB structure adds noise and complicates, if not limits, a large gain-bandwidth design. Third, increasing the number of devices used in a given topology also increases the parasitic capacitances in that topology. These parasitic capacitances need to be driven, thereby increasing the power used. Fourth, using CMFB structures also burns additional power.
As a result, it would be desirable to provide a fully differential mixer and class AB post-mixer amplifier system for SDR applications that is flexible, low noise, high linearity, high frequency, and low power. Furthermore, it would also be desirable to provide a radio frequency integrated circuit (RFIC) architecture that includes a post-mixer amplifier with improved characteristics.